1. Field of the Invention
This invention relates to microwave signal generation and more particularly relates to laser-assisted field emission generation of a microwave signal.
2. Description of the Related Art
The increasing performance demands of high-speed communications require the generation of electromagnetic signals at ever-higher microwave frequencies. Yet the physical constraints of materials and electromagnetic radiation have limited the frequencies of microwave signals. The output power of both vacuum tube- and semiconductor-based signal generators fall off sharply at frequencies above 1 terahertz.
The operating frequencies of electronic devices have been increased by taking advantage of the higher switching speeds of optoelectronic devices. The Auston Switch uses pulsed lasers to modulate the conductivity of a photoconductive substrate such as Gallium Arsenide (GaAs). The laser pulse excites electrons from a valence band to a conduction band, changing the substrate from an insulator to a conductor. Auston Switches have switching times of about 500 fs, allowing them to generate extremely narrow electrical pulses or high-frequency signals.
Lasers have also been used to modulate the current in field emission devices. In field emission, an applied electric field reduces the potential barrier at the surface of a metal or semiconductor. When the potential barrier is reduced to be near the Fermi level of the electrons, the electrons “tunnel” from the metal or semiconductor. The tunneling electrons create an electric current. The tunneling electron current can be modulated by laser radiation. The response time of tunneling electron current to a laser pulse can be as brief as 2 fs, less than one percent of the response time of the photoconductive substrate in an Auston Switch, making laser-modulated field emission-based devices ideal for microwave signal generation.
A laser-assisted field emission signal generator must drive a load with a typical input impedance of about 50 Ω. Yet the impedance of the field emission device is much higher. Unless the high impedance of the field emission device is matched with the low impedance of the load, a laser-assisted field emission signal generator cannot produce a useful signal.
What is needed is a process, apparatus, and system that generates a microwave signal using a laser-assisted field emission device. Beneficially, such a process, apparatus, and system would generate a high-frequency, tunable microwave signal. The process, apparatus, and system would further couple the high impedance of the field emission to a lower output impedance.